Taurine uptake in isolated retinas of normal rats and rats with hereditary retinal degeneration

Abstract

Kinetic analysis reveals two processes for the uptake of taurine in the isolated rat retina. One process, designated as a high affinity uptake mechanism, has an apparent Michaelis constant ( K m ) of 50 μ m and a maximum velocity of uptake ( V max) of 0·3 nmol/mg dry wt/min. The other process, designated as a nonsaturable uptake mechanism, has a rate constant of 0·53 nmol/mg dry wt/min/m m taurine in the medium. Retinas from normal rats, studied at ages 22–180 days, and from Royal College of Surgeons (RCS) rats with normal or reduced numbers of photoreceptor cells, studied at ages 22–45 postnatal days, show both mechanisms at a time when photoreceptor cell function is detectable in vivo with the electroretinogram (ERG). Retinas from 180-day-old photoreceptorless RCS rats retain the nonsaturable uptake mechanism for taurine but do not show the high affinity uptake mechanism at a time when the number of photoreceptor cells is greatly reduced and no photoreceptor function can be detected with the ERG. The high affinity uptake mechanism is selectively lost in isolated normal rat retinas with mechanical disruption of photoreceptor cells and is inhibited to a greater extent than the nonsaturable mechanism by ouabain and reduced temperature. These studies support the idea that the high affinity uptake mechanism for taurine in rat retinas depends on the presence of viable photoreceptor cells.